Conventional glass batching processes are illustrated as a flow diagram in FIG. 1. Typical glass batching usually involves transferring raw materials directly from storage silos into a weigh hopper, weighing the raw materials according to a weight percent (wt %) batch recipe, adding a specified amount of cullet, and mixing the raw batch and the cullet in a large scale mixer. In some cases, the mixer itself functions as a final check-scale for the batch recipe. From the mixer, the mixed batch materials are transferred to one or more hoppers positioned adjacent the end of a glass furnace (melter) where the mixed batch is introduced into the melting tank. Similar batching techniques are nearly universally employed in various glass producing industrial settings, including container glass, fiber-glass, and float glass manufacturing facilities.
After the mixed batch is added to the furnace (melter), uncontrolled reactions are allowed to occur in melter at various temperatures, both among the batch raw material components and between the batch raw material components and resident melt, until a substantially homogenous melt is eventually achieved. The time required for sufficient melting, homogenization and fining is related to the total residence time, or the time that the melt resides within the melter tank before being formed into the desired glass product.
FIG. 2 is a schematic illustration showing the reaction paths that the raw material batch components typically follow when reacting with each other and with the melt already present in the furnace, and FIG. 3 is a schematic illustration showing the conventionally uncontrolled melting stages as the newly added batch melts. See also, for example, F. E. Woolley, “Melting/Fining,” Ceramics and Glasses, Engineered Materials Handbook, Vol. 4, ASM International, 1987, pp. 386-393, the entirety of which is incorporated herein by reference.
That is, once the batch is introduced to the furnace, several reactions take place that almost immediately segregate the batch. In float glass production, for example, sodium carbonate (Na2CO3), calcium carbonate (CaCO3), sodium sulfate (Na2SO4) and quartz (SiO2) are the most commonly used major raw materials. When water has not been added in an effort to reduce batch segregation in the storage hopper, the first reaction is usually the formation of a eutectic liquid by the reaction of Na2CO3 and CaCO3 at a temperature of around 785.degree. C.
As shown in FIG. 2, Na2CO3 and CaCO3 react along reaction Path 1, creating a low viscosity eutectic liquid with a quantity of un-reacted CaCO3. This low viscosity eutectic liquid reacts with residual CaCO3 and quartz along reaction Path 2 to eventually achieve the overall composition of the glass dictated by the batch recipe. An example of a typical float glass composition is approximately 73.5 wt. % SiO2, 12.3 wt. % CaO, and 14.2 wt. % Na2O.
Similar reactions are observed between Na2CO3, CaCO3, and Na2SO4. In this case, the eutectic liquid is composed of molten salts having a very low viscosity. That is, the eutectic liquid flows easily, and exhibits flow properties similar to those exhibited by water, which has a viscosity in a range of 1 to 4 mPa·s, or 1 to 4 centipoise. The eutectic liquid reacts with the quartz to eventually provide a homogeneous glass of the desired composition. The formation of this eutectic liquid, however, can increase the tendency for batch segregation and effectively reverse the efforts of batch mixing.
Similar reactions occur in container glass compositions, and in the case of fiber-glass production, borates exhibit similar problems in the initial stages of melting. This segregation process leads to the formation of large-scale domains, or agglomerates, of nearly pure silica that then require excessively long residence times for dissolution into the surrounding liquid melt. This initial segregation then requires re-homogenization within the glass tank prior to forming.
Direct evidence of “de-mixing” can be seen in a glass tank during the melting process. Agglomerations (scaled on the order of cm in length) of batch raw materials, commonly referred to in the industry as batch logs, can be seen in various states of melting in the glass tank. Moreover, the phenomenon of large-scale batch segregation in the melter tank is commonly seen in finished glass in the form of defects such as stones, which are mostly composed of undissolved quartz; seeds, which are bubbles that are not liberated from the melt during fining; and cord lines, which are optical distortions caused by local differences in composition. These defects are direct evidence of off-composition glass due to batch de-mixing or incomplete re-mixing that decrease the overall material efficiency and reduce the quality of the final product. Industrial observations are further supported by technical publications, which also recognize that batch segregation is commonly observed in commercial production. Despite the fact that batch segregation in the glass tank and the potential defects that can result therefrom are recognized in the industry, and despite a long felt need to reduce this undesirable behavior and improve melting efficiency and overall quality, the glass industry has not yet successfully addressed these issues with a viable commercial solution.
As mentioned above, material efficiency in glass making is related to reducing losses due to defects such as stones, seeds, and cord lines. Stones are silica particles or agglomerates that have not fully reacted with the melt. This type of stone can be reduced by reducing segregation of refractory silica from flux materials. Seeds, which are bubbles that result from incomplete fining, can be reduced by maximizing the evolution of volatiles early in the melting process and by reducing air trapped in pore spaces. While cullet from some defective glass can be recycled through the process (though glass with stones cannot be recycled), it is more efficient to reduce in-house cullet from defective glass.
In large scale commercial glass production (e.g., float glass, container glass, and fiber-glass) where the melting tank volumes are considerably greater (accommodating volumes on the order of tons of molten glass), in situ melt mixing is accomplished by convection currents within the tank and by the movement of evolved gases from decomposition of raw materials. While some mixing and fining is required to remove gaseous bubbles, the expensive and energy intensive processes to improve the mixing of the molten batch can also be attributed to large scale segregation of batch materials. Alternatively, a mixture may be combined that will retard or substantially prevent segregation of a low viscosity phase by creating a higher viscosity suspension with solids present until a sufficient reaction temperature is reached where the solids dissolve. For instance, combinations of CaO and B2O3 can have quite low viscosities at moderate temperatures relative to the glass melter used. When the selectively batched first combination contains suitable amounts of more refractory silica, clay, or other refractory materials, the viscosity of the system is significantly raised by the suspension of unreacted solids from the granule in the forming liquid composition. The system is held together by capillary forces while the temperature rises. As the temperature increases, more silica and/or alumina reacts with the mixture yielding a higher viscosity liquid phase (relative to the composition without the additional silica and/or alumina) thus preventing or inhibiting segregation of components, such as by a free-flow mechanism or the like. The greater contact surface between the calcium borate liquid phase and the silica material further enhances the reaction kinetics that lead to the final desired glass composition.
Considering that physical mixing is but a minor factor, the efficiency of the melting process is therefore directly related to diffusion or reactions at the quartz-liquid interface. Quartz dissolution is limited by the initial reaction of quartz with the low viscosity eutectic liquid. As the melting progresses, the quartz interacts with a liquid that is steadily increasing in silica content and subsequently, viscosity. Therefore, high temperatures are needed within the melting tank to ensure reasonable diffusion rates and reasonable homogeneity. As mentioned above, the residence time of the material in a tank is determined by the time it takes for the batch materials to completely melt and for the resulting liquid to homogenize. In a continuous production situation, the mass of molten glass in the furnace is held constant, and commercially, the minimum mean residence time is of the order of 24 hours of production for container furnaces and 72 hours for float glass furnaces with roughly half of this time devoted to melting, with the other half devoted to fining.
One attempt to improve the batch melting process involved reducing the addition of carbonate and quartz in the raw (unmixed) form. Experiments were conducted using synthetic diopside (CaO.MgO.2SiO2) instead of a mixture of CaCO3, MgCO3, and quartz. The results showed that the time required to completely dissolve the original batch (i.e., the batch free time) was reduced depending on temperature, and there was also a reduction in fining time. These improvements were attributed to a reduction in the amount of quartz that needed to be dissolved. See, for example, C. C. Tournour and J. S. Shelby, “Effect of Diopside and Wollastonite on the Melting of Soda-Lime-Silicate Glasses,” Ceramic Engineering and Science Proceedings, edited by J. Kieffer, American Ceramic Society, 21 [1], 263-273 (2000), the entirety of which is incorporated herein by reference.
It is also conventionally believed that melting is promoted by keeping the viscosity low. As described above, however, the uncontrolled production of low viscosity liquids during the melting process contributes to undesirable batch segregation. Although a melt that fosters lower viscosities overall may improve quartz dissolution and diffusion rates during melting, these benefits can only be achieved after the highest melting point batch components are sufficiently melted and any batch agglomerates are fully reacted in the melt. Thus, in order to improve melting efficiency and reduce the above-described problems associated with de-mixing and segregation, substantial improvements with respect to controlling the glass batch melting behavior are desired.
Another problem with conventional glass making technology lies in the amount of energy required to maintain a continuous glass melting operation, and the environmental impact of the use of fossil fuel to provide this energy. Fuel can constitute 25-30% of the cost of manufacturing float glass. The volatility of fuel prices can, of course, at times increase this proportion without warning.
Nationwide, the U.S. glass industry uses in excess of 250 trillion BTU annually to produce approximately 21 million tons of glass products; approximately 80% of this energy is supplied by natural gas. Melting one ton of glass should theoretically require only about 2.2 million BTU, but in reality it can range from 4.7 to 6.9 million BTU per ton due to losses and inefficiencies. Because 80% or more of the overall energy used in container glass, fiber-glass, and float glass manufacturing is needed to operate the melting and fining operations, an energy reduction in glass manufacturing through more efficient melting would be desirable. For example, if a float glass plant producing 400 tons/day of flat glass runs 365 days/year, even the most efficient natural gas-fired plant (4.7 million BTU/ton) consumes approximately 686 billion BTU/year, or 686 million cubic feet of natural gas. See, for example, U.S. Department of Energy, Office of Industrial Technology, 1997, and “Integrated Pollution Prevention Control (IPPC),” Reference Document on Best Available Practices in the Glass Manufacturing Industry, European Commission, Institute for Prospective Technological Studies, Seville, 2000, the entireties of which are incorporated herein by reference.
Pollution prevention and the considerable costs associated with regulatory compliance, as well as improving the overall energy and material efficiency are critical for reducing the negative environmental impact of glass manufacturing and for making glass manufacturing more economically competitive. For example, a typical float glass plant must spend an average of $2 million dollars for new environmental control systems and about 2.5% of total manufacturing costs on compliance. (See, for example, “Glass: A Clear Vision for a Bright Future,” U.S. Department of Energy, 1996, the entirety of which is incorporated herein by reference). Thus, a reduction in 10% of the natural gas use in a typical float plant would result in a savings of approximately $285,000 per year in natural gas (assuming $5/MMBtu). Moreover, reductions in compliance costs associated with additional chemical treatments and operational implementations aimed at reducing pollutant emissions from combustion reactions could also be realized in conjunction with a reduction in the amount of fuel consumed.
Air pollutants emitted from glass industry include:    1) Nitrogen oxides (NOx)    2) Sulfur oxides (SOx)    3) Carbon monoxide (CO)    4) Carbon Dioxide (CO2)
Fossil fuels used for combustion are the typically the sources of NOx and some COx. The decomposition of carbonate and sulfate raw materials contributes COx and SOx emissions, respectively. Reducing the residence time, however, reduces the amount of fuel burned per unit of glass produced and improves energy efficiency, which also fosters reduced amounts of emissions such as NOx and fuel-derived CO2 and CO per unit of glass produced.
Residence time is related to the time required to fully melt all of the batch components, and is particularly dependent upon the amount of high-melting point batch components (e.g., silica) in the batch recipe. Although it would be desirable to eliminate free quartz as a raw material additive due to its slow reactivity and high melting point, quartz remains an abundant and economical source of silica, which is a major component of many commercial glass systems. Therefore, it would be more desirable to reduce the amount of free quartz added by obtaining a portion of the silica from selectively combined binary or ternary mixtures that are either pelletized together, pre-reacted or pre-melted prior to batching and being introduced into the resident melt, which is heretofore unknown in the glass industry.
Thus, it would be desirable to provide a method for controlling the melting behavior (i.e., reaction paths) of glass batch components within a resident melt to improve melting efficiency, such that the improved melting efficiency enables a decrease in energy usage, reduces the need for chemical fining agents that contribute to air pollutants and raw material cost, decreases pollution while ultimately producing higher quality, lower cost glass products and reduces the occurrence of batch de-mixing and segregation in early melting stages.